JPH07316838A - Formation of heat insulating coating film - Google Patents

Abstract

PURPOSE:To provide a method for forming a heat insulating coating film on the surface of an iron member, especially a method for forming a heat insulating coating film having such improved film strength that the cracking and peeling of the iron oxide-based coating film at a high temp. are suppressed. CONSTITUTION:When a heat insulating coating film is formed, a ceramic compsn. consisting of powder based on one or more among Fe2O3, Fe2O3, and FeO and 15-60wt.% chromium oxide powder or 10-4Owt.% Cr powder based on the total amt. of the powders is fired to form a multiple oxide having a spinel structure or a ceramic compsn. consisting of powder based on one or more among Fe2O3, Fe3O4 and FeO and chromium oxide powder obtd. by subjecting 10-40wt.% Cr powder to oxidation treatment under conditions in which an unoxidized part is left is fired.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat insulating coating formed on the surface of an iron member, and more particularly to a method for forming a heat insulating coating with improved strength such as cracking and peeling of the iron oxide coating at high temperatures.

[0002]

2. Description of the Related Art Iron-based members are surface-treated to provide heat insulation and oxidation resistance when they are repeatedly used at high temperatures. Of these, most of the coating treatments are mainly made of ceramics. However, the bonding strength between the metal and this ceramic is not sufficient, and especially for members used in high temperature environments for a long time such as automobile parts, the underlayer oxidation generated between the surface coating layer and the base material. This phenomenon causes a decrease in the bonding strength at the interface and eventually leads to peeling of the coating layer.

Generally, in the joining of a metal base material and a ceramic, since the joining strength is a problem due to the porosity of the ceramic, a stress is applied at the time of melting by a mechanical method and the sealing action is utilized to form an interface. It has been made to improve strength. Even in this method, it is difficult to obtain sufficient strength because the wettability of both is limited. Further, even if an attempt is made to improve the wettability by another chemical method, it is difficult to sufficiently increase the contribution to the bonding strength in the relationship between the base material and the gas accompanying the chemical reaction. (For example, Material Engineering Dictionary, p2463, vol. 4, 198
6)

Further, JP-A-61-163282 proposes to jointly coat ceramics on parts such as exhaust system parts of an internal combustion engine which are required to have heat resistance and heat insulation. In addition, Japanese Patent Laid-Open No. 61-26781 discloses a coating for forming an alloy and a ceramic layer in which the average linear expansion coefficient of each layer is regulated as a laminated body composed of two or more layers, and a method for producing the same.

Although it is considered that the ceramic layers in these prior arts have sufficient heat resistance in a use environment of about 800 ° C., in recent years, the air-fuel ratio of an automobile engine has become leaner. As a result, the temperature of exhaust gas rises, and in order to cope with this, it is necessary to develop a coating having heat resistance of more than 800 ° C.

[0006]

In a coating material containing an iron-based oxide such as Fe ₂ O ₃ as a main raw material, at a high temperature of 800 ° C. or higher, the sintering of the Fe ₂ O ₃ powder proceeds, and finally, By this sintering, the coating film shrinks, cracking and peeling occur in the coating film, and the coating film strength decreases. Therefore, the shrinkage at the interface between the metal member and the surface coating layer is prevented, and the bonding strength is improved,
Further, a technique for forming a coating layer that exhibits sufficient heat insulation and oxidation resistance is desired.

An object of the present invention is a coating material for an iron-based oxide such as Fe ₂ O _3, which is formed by sintering a metal member surface and the main raw material, at a high temperature, Fe ₂ O ₃ powder A heat-insulating coating that prevents shrinkage during the sintering stage and during use, does not cause crack peeling due to this shrinkage, suppresses deterioration of binder strength at high temperatures, and prevents deterioration of coating strength. A forming method is provided. Another object of the present invention is to make the Cr powder, which has been difficult to be made into a slurry, to be easily made into a slurry by a pretreatment so that it can be uniformly applied to an iron oxide powder, and by sintering, from Fe and Cr. Is to form a film that produces a complex oxide having a spinel structure, and to develop the heat insulating property and strength of the film.

[0008]

[Means for Solving the Problems] The above object is to provide a heat insulating film.
Of forming Fe,₂O₃, Fe₃O_Four, FeO
Powder containing at least one of the above as the main component, and the total amount of powder
15 to 60 wt% of chromium oxide powder
Of a heat-insulating film characterized by firing ceramic components
It is achieved by the formation method. In addition, the method of forming the thermal insulation film
And Fe ₂O₃, Fe₃O_Four, FeO at least
Also, a powder containing at least one kind as a main component, and 1 for the total amount of powder
Ceramic component consisting of 0-40 wt% chromium powder
By the method of forming a heat-insulating film, which is characterized by firing
Is also achieved.

Furthermore, a method for forming a heat-insulating film, comprising the steps of F
e ₂ O ₃ , Fe ₃ O ₄ , and powder containing at least one of FeO as a main component, and 10 to 40 wt% based on the total amount of powder.
It is also achieved by a method for forming a heat-insulating film, which comprises firing a ceramic component consisting of a chromium oxide powder obtained by subjecting the chromium powder described above to an oxidation treatment under conditions where an unoxidized portion remains.

[0010]

According to the present invention, the first feature of the present invention is to fire a ceramic component composed of a powder containing iron oxide as a main component and a chromium oxide powder. By the reaction of the iron oxide and the chromium oxide at the time of sintering, a double oxide having a spinel structure is produced, and the film strength is improved. Further, the present invention is to fire a ceramic component consisting of iron oxide and chromium powder, the reaction at the time of sintering of the iron oxide and chromium to produce a double oxide having the spinel structure,
The film strength is improved. Further, when chromium is oxidized, it expands to offset the sintering contraction of iron oxide, and cracking is prevented even when sintering progresses during firing or due to the heat load during use.

Further, according to the present invention, a ceramic component composed of iron oxide and chromium oxide powder obtained by oxidizing chromium powder under conditions where an unoxidized portion remains is fired. That is, by partially oxidizing the chromium powder, slurry formation can be facilitated.

[0012]

[Supplementary Explanation of Means for Solving the Problems] Hereinafter, the features of the process of the present invention will be described more specifically. Regarding the first characteristic point of the present invention, more specifically, Fe
_The proportion of at least one of ₂ O ₃ , Fe ₃ O ₄ , and FeO as the main component, and the proportion of at least one of Cr ₂ O ₃ , CrO ₃ , and FeCr ₂ O _{4 in} the ceramic powder is 15 to 60 wt%. %, And the balance substantially consists of a binder to sinter the ceramic powder.
At this time, 10 to 40 wt% of the added chromium amount is equivalent to 15 to 60 wt% as the chromium oxide amount of the formed coating film. (Chromium increases weight by 1.5 times due to oxidation)

As the binder, inorganic phosphate,
Water glass silicate, silica sol, alumina sol and the like are preferable. The second feature is, more specifically, Fe ₂ O ₃ and F.
A step of producing a slurry composed of a powder containing at least one of e ₃ O ₄ and FeO as a main component, Cr powder (particle size 1 to 20 μm) of 10 to 40 wt% based on the total weight of the raw material powder, and a binder. A step of applying this slurry onto a member, and a step of baking the applied film at a temperature of 800 ° C. to 1000 ° C. for 1 hour or more.

If the amount of added Cr is less than 10% by weight, the amount of sintering shrinkage of iron oxide is significantly larger than the amount of oxidative expansion of chromium, which is insufficient for offsetting the shrinkage, resulting in cracking of the coating film and lowering the strength. On the other hand, if it exceeds 40 wt%, the oxidative expansion amount of chromium is significantly larger than the sinter shrinkage amount of iron oxide, and conversely, coating cracking occurs due to excessive expansion and the strength decreases. Incidentally, only reason for said Cr in the present invention, volumetric expansion during oxidation can offset the sintering shrinkage of the _{Fe 2 O 3, Fe 2 O} 3
This is because a spinel structure is generated and the strength is also improved. As the optimum condition, the Cr grain size is preferably 1 to 20 μm. If it is less than 1 μm, the oxidation at the time of temperature rise is too fast,
It oxidizes rapidly before the reaction start temperature with ₂ O _3, and it does not match the contraction timing of iron oxide. In this case, it does not contribute to the effect of offsetting the dimensional change and the enhancement of strength by spinel formation. 20μ
If it exceeds m, the reaction with Fe ₂ O ₃ does not sufficiently occur because it is not uniformly dispersed in the coating film. Particularly with respect to the third feature of the present invention that partially oxidizes the powder, if it is not less than 1 μm,
Control of partial oxidation is difficult.

The firing conditions are preferably 800 to 1000 ° C. If the temperature is lower than this range, the sintering of iron oxide will be insufficient, and if it is too high, reductive decomposition of iron oxide will occur and the film strength will decrease. That is, the iron oxide composition is Fe ₂ O ₃ → Fe ₃
It changes from O ₄ → FeO → Fe. The firing time is preferably 1 hour or more. If it is less than 1 hour, the reaction between iron oxide and added Cr does not sufficiently occur. If the firing atmosphere is particularly an inert atmosphere, a complex oxide having a spinel structure is likely to be formed. Regarding the first feature of the present invention in which the starting material is chromium oxide, if it is less than 15 wt%, it is difficult to exert the effect due to the formation of the double oxide having a spinel structure. Therefore, the values are limited to the above values.

The firing atmosphere may be air or an inert atmosphere. However, if it is desired to further improve the strength, firing in an inert atmosphere is desirable. By firing in an inactive manner, Cr receives oxygen only from Fe ₂ O _3, so that the bond of Cr—Fe—O is easily taken and a spinel structure is easily formed.

The third characteristic point is that the chromium powder to be added (particle size 1 to 20 μm) is fired under the optimum conditions of the shaded area from the relationship between the firing temperature and the time shown in FIG. It is a method for pretreatment of chromium powder, characterized by oxidizing the surface. In this case, if the particle size is less than 1 μm, most of the powder is oxidized and the effect of Cr addition is impaired.

FIG. 4 shows the relationship between firing time and temperature.
In the lower left area deviating from the shaded area, there is little oxide layer on the surface of the Cr powder, and the Cr powder aggregates in the slurry, making it impossible to form the slurry. Further, in the upper right region outside the shaded portion, most of the powder is oxidized, and the effect of Cr addition is impaired.

In addition to the method of the present invention, as a method for making a slurry, Fe ₂ O _{3 is formed} around Cr powder.
There is a method of adhering powder or the like by a granulation method to improve wettability with a slurry. With this method, it is industrially difficult to use Cr powder having a particle size of 20 μm or less. Also, C
There is a method of coating stearic acid, a SiO ₂ water-repellent coating agent, etc. around the r powder to improve the wettability to the slurry. In this method, stearic acid and S
Depending on the amount of coating agent such as iO ₂ , Fe ₂ O ₃
/ Cr and Cr ₂ O ₃ may be inhibited, and the treatment is costly and unsuitable industrially.

[0020]

EXAMPLES The present invention will be described in detail below with reference to the accompanying drawings by way of example embodiments and comparative examples of the present invention. Example 1 As an example of the present invention, the present invention was applied to a cast iron exhaust manifold for automobiles (hereinafter abbreviated as Ex manifold). The film forming process of the product of the present invention is as follows. Roughening the inner surface of cast iron Ex manifold for automobile by shot blasting
Ni (50 wt% Fe-50 wt% Ni) alloy powder (grain size 1
0 to 20 μm) 70 parts by weight of Cr powder (particle size 1
˜2 μm) 30 parts by weight, butanol is further added to this, and the mixture is sufficiently stirred to obtain a raw material slurry. The viscosity at this time is preferably 1000 to 4000 cps. The slurry was poured into an Ex manifold, coated so that the film thickness was 100 μm, dried at 200 ° C., and then kept at 900 ° C. in an Ar atmosphere for 5 hours. The intermediate layer can be formed by the above processing.
20 parts by weight of Cr powder (particle size 1 to 2 μm) was added to 80 parts by weight of Fe ₂ O ₃ powder (particle size 1 to 400 μm),
To this, 80 parts by weight of an aluminum phosphate aqueous solution (concentration 30%) is added and sufficiently stirred. This Cr powder is 400 ℃ in advance
It was used after firing for 2 hours. This slurry is poured into an Ex manifold having an intermediate layer and dried at 300 ° C. Repeat the above and steps for 3 cycles, 500
An Fe ₂ O ₃ layer having a thickness of μm was obtained. Then, it was kept in an air atmosphere at 850 ° C. for 5 hours and fired to form a heat insulating film.

The film characteristics of the film of this example were adjusted.
I checked. The Ex manifold was cut and the cross section was examined. That
As a result, defects such as cracks were not recognized. In addition, off
An EPMA line analysis of the cut product was performed. As a result, the outermost layer
In the intermediate layer, the Cr component is inclined,
It was found to be firmly joined. Also, the middle class and mother
Ni and Fe components are inclined with respect to cast iron, and similarly strong
It turns out that it is joined to. Next, the ingredients of the outermost layer
It was investigated by X-ray diffraction. As a result, Fe₂O ₃, Cr
₂O₃And FeCr₂O_FourWas measured. This result
In the firing according to the claims, iron oxide reacts with chromium.
And form their respective oxides, and the complex acid of both
It is thought that it forms a compound and contributes to the strength improvement.
Be done.

Example 2 In this example, in order to investigate the relationship between the mixing ratio of iron oxide powder and chromium powder and high temperature shrinkage, iron oxide powder / chromium powder mixed powder products with various mixing ratios were prepared, Aluminum phosphate binder is added to this to make a slurry, which is 5 × 5 ×
It was poured into 10 concave molds and dried at 300 ° C. to obtain 5 × 5 × 10 test pieces. 900 this
It was held in a furnace at 100 ° C. for 100 hours, and its dimensional change was investigated.
The result is shown in FIG. 10 which is the claim of the present invention
It can be seen that the dimensional change is small in the 40% Cr region. In the region of 10% or less, the shrinkage is remarkable, but this is due to the shrinkage accompanying the sintering of iron oxide. Also, 4
In the region of 0% or more, on the contrary, it significantly expands, but this is due to the oxidation expansion of Cr. Cr addition 10
The line in the figure becomes relatively constant in the range of up to 40%, because the addition of Fe in the above range causes positive formation of FeCr ₂ O ₄ .

Example 3 In this example, the base metal was cast iron and the cast iron was coated. The coating method in this case was the same as in Example 1. FIG. 1 is a schematic diagram of the cross-sectional structure in the dry state before firing, and FIG. 2 is a schematic diagram of the cross-sectional structure after the firing. In FIG. 1, the metal Cr3 and the iron oxide 2 are uniformly dispersed and are retained even after firing. In FIG. 2, the complex oxide 6 (FeCr ₂ O ₄ ) having a spinel structure is iron oxide 2 (Fe ₂ O ₃ ) and chromium oxide 5 (Cr ₂ O ₃ ).
It is formed thick at the interface with. In this example, the cast iron base material (50 x
50 × 5) and the coating strength was measured by a shear test. The results are also shown in Table 1. From the results of Table 1, it was confirmed that the amount of chromium added is within the scope of the claims of the present invention.

[0024]

[Table 1]

Example 4 In this example, the Fe-based alloy sintered part was used as an intermediate layer, and the intermediate layer was coated. The coating method in this case was the same as in Example 1. On a shot-blasted cast iron base material (50 × 50 × 5), 70 parts by weight of a FeNi powder, 30 parts by weight of a slurry of Cr powder and butanol were applied and fired at 900 ° C. for 5 hours in an argon atmosphere, Layers were obtained. On this, the same slurry as in Example 3 was applied and fired, and similarly the coating adhesion strength (strength between the outermost surface layer and the intermediate layer) was measured. FIG. 3 is a schematic diagram of the cross-sectional structure after the firing. In this figure, the iron-based alloy sintered portion 7 is interposed as an intermediate layer, and the ceramic coating layer is laminated on the intermediate layer. The results are also shown in Table 1. It can be seen that, by interposing the intermediate layer of the present invention, the adhesion strength is improved as compared with the case of directly coating on cast iron.

Example 5 In the same manner as in Examples 3 and 4, only the metal powder material in the slurry was changed and the superiority of Cr was confirmed. The results are shown in Table 2. From this result, it is understood that Cr is preferable as the added metal powder. As shown in Table 2, when Fe powder is added, the oxidizing property of Fe is too strong, so that the film expands and the strength decreases. On the other hand, Ni, Co, and Cu have poor oxidizability and reactivity with Fe ₂ O _3, and cannot be expected to have improved strength.
Therefore, the additive metal powder is limited to Cr in consideration of matching with Fe ₂ O ₃ . Table 2 also shows an example in which the product of the present invention is manufactured by the method of adding Cr ₂ O ₃ in advance.
In this case, since the oxidative expansion of Cr cannot be expected, the strength is lowered, but the strength is less reduced as compared with other metal powder-added products and the like due to the formation of spinel.

[0027]

[Table 2]

Example 6 In this example, the conditions for forming a slurry of Cr powder were examined. A slurry was prepared by using Cr powder having different calcination conditions as a raw material, and the slurry was passed through a sieve having a mesh size of 50 μm to measure the amount deposited on the sieve. At this time, deposition rate = deposition weight / raw material Cr weight × 1
The state of agglomeration of the raw material chromium was compared by using 00 (%). In this example, the Cr powder after calcination was subjected to X-ray diffraction, and the chromium oxide content ratio was also measured. The results are shown in Table 3. From this result, under the oxidizing conditions other than the region surrounded by the thick line in FIG.
It was confirmed that the powder aggregated and could not be slurried, or was completely oxidized, which was contrary to the original purpose.

[0029]

[Table 3]

[0030]

INDUSTRIAL APPLICABILITY The present invention enables formation of a film made of a double oxide having a spinel structure and having excellent heat insulating properties and strength on the surface of a metal member. Further, in the case of a ceramic coating containing Fe ₂ O ₃ as the main raw material powder, an air atmosphere at 800 ° C. or higher or a low oxygen partial pressure atmosphere (for example, exhaust gas atmosphere)
In, the reduction reaction with _{"6Fe 2 O 3 → 4Fe 3 O} 4 + O 2 ", powder undergoes a density change, but still had also collapse of the powder, the Fe ₂ O ₃ as in the present invention Cr Since FeCr ₂ O ₄ is formed by the addition of Al, the stability at high temperature is excellent, and the above-mentioned disintegration of the powder is not observed.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a cross-sectional structure of a coating layer after drying iron oxide and Cr according to the present invention.

FIG. 2 is a schematic diagram of a cross-sectional structure of a coating layer obtained by firing the one of FIG. 1 according to the present invention at 850 ° C. for 5 hours.

FIG. 3 is a schematic diagram of a cross-sectional structure of a coating layer having an intermediate layer of an Fe-based alloy sintered portion according to the present invention.

FIG. 4 is a diagram showing the relationship between the firing time and the firing temperature for the pretreatment of Cr according to the present invention.

FIG. 5 is a diagram showing the relationship between the Cr addition rate and high temperature shrinkage according to the present invention.

[Explanation of symbols]

1 ... Inorganic binder 2 ... Fe ₂ O ₃ 3 ... Cr 4 ... Base metal 5 ... Cr ₂ O ₃ 6 ... FeCr ₂ O ₄ 7 ... Fe alloy sintered part

Claims (3)

[Claims] 1. A method for forming a heat insulating coating, comprising Fe ₂ O
Thermal insulation characterized by firing a ceramic component consisting of a powder containing at least one of ₃ , ₃ , Fe ₃ O ₄ , and FeO as a main component and a chromium oxide powder of 15 to 60 wt% with respect to the total amount of powder. Method of forming a film. 2. A method for forming a heat insulating coating, comprising Fe ₂ O
A heat-insulating film characterized by firing a ceramic component composed of a powder containing at least one of ₃ , ₃ , Fe ₃ O ₄ , and FeO as a main component and a chromium powder of 10 to 40 wt% with respect to the total powder amount. Forming method. 3. A method for forming a heat insulating coating, comprising Fe ₂ O
A powder containing at least one of ₃ , ₃ , Fe ₃ O ₄ , and FeO as a main component, and a chromium oxide powder obtained by oxidizing 10 to 40 wt% of chromium powder with respect to the total amount of powder under conditions where an unoxidized portion remains. A method for forming a heat-insulating film, which comprises firing a ceramic component consisting of